In performing biological tests and analyses, it is often necessary to dispense a series of liquids such as reagents into a test area, e.g., a slide. For example, in an immunoassay of tumor tissue, a thinly sliced section of the tissue is placed on a slide, rinsed and covered with an evaporation inhibitor. Then a measured amount of a primary antibody solution (e.g., 100 .mu.l) is dispensed onto the slide. The antibody solution passes through the evaporation inhibitor and comes into contact with the tissue section. The primary antibody and rinse solution remaining on the slide (e.g., .about.300 .mu.l) are mixed, by directing a stream of air onto the slide, and are incubated for a period of time. The slide is washed and the process is repeated with an enzyme-labeled secondary antibody solution. After another washing, an enzyme substrate solution (indicator), a color enhancer, and a counterstain are added to the slide in sequence. Before each of these liquids is added, the slide is washed and a fresh layer of evaporation inhibitor is applied.
In a process such as the foregoing, many of the reagents must be deposited on the slide in precisely measured small amounts (often expressed in .mu.l). The process is highly labor intensive and can be variable if performed manually, and therefore a number of devices have been proposed for automating all or a portion of the process.
Application Ser. No. 07/488,601, filed Mar. 2, 1990 co-owned with this application and incorporated herein by reference, describes a reagent carousel which cooperates with a slide support carousel to apply a sequence of preselected reagents to each of the slides with interposed mixing, incubating and rinsing steps. The reagent carousel holds a plurality of reagent containers and includes a drive means for positioning the appropriate reagent container over one of the slides in the slide carousel. Air cylinder or equivalent actuators make contact with the individual reagent containers and thereby cause reagent to be delivered on to the slides.
The reagent container described in the above-mentioned application Ser. No. 07/488,601 is a small bottle capped with a microdelivery pump/valve. After the pump/valve is primed, the bottle is inverted and placed on the carousel above the slide area. The actuator is positioned above the bottle and, when activated, forces the bottle downward, dispensing a predetermined volume of reagent.
Liquid dispensing arrangements such as this have several limitations. First, they frequently include a metal compression spring in the path of the fluid. The metal spring may react with several of the liquids dispensed (e.g., salts), and metal ions which enter the reagent may act as a catalyst for redox reactions which will destabilize indicators and proteins and will create problems with the specific chemical reactions, generally causing a loss of activity and a loss in sensitivity of the assays. Second, the bottle is not vented and develops an internal vacuum as the reagent is dispensed. To avoid generating a complete vacuum, the bottles are normally only half filled. Third, a certain amount of air remains in the pump chamber after the priming process. This air may deflect the reagent stream sideways or cause it to spray out of the nozzle during the initial dispensing operations, causing the volume of liquid delivered to be unpredictable. Moreover, as the pressure inside the bottle falls during succeeding dispensing operations, this air migrates toward the interior of the bottle, causing the volume of liquid dispensed to increase. Fourth, a drop of liquid forms at the tip of the nozzle after each dispensing operation, and may fall off as the carousel rotates. This can create problems if, for example, the drop lands on another slide. Finally, some of the bottles must be opaque to protect photosensitive reagents, and the operator is unable to tell how much reagent remains in the bottle.
The liquid dispenser of this invention overcomes all of these problems.